1. Field of the Invention
The present invention relates to an apparatus for purifying exhaust gas emitted from internal combustion engine and, in particular, to an apparatus for purifying NOx (nitrogen oxide) emitted from a motor vehicle.
2. Description of Related Art
In order to purify an exhaust gas emitted from an internal combustion engine such as a motor vehicle, it is usual to reduce NOx in the exhaust gas by the use of NOx purifying catalyst carrying a noble metal. However, it is necessary to use a larger amount of noble metal, to sufficiently purifying NOx at a lower temperature, as the NOx purifying catalyst generally does not have much NOx purifying ability at the lower temperature. Using the larger amount of noble metal increases costs and does not always provides a satisfactory result.
Japanese Unexamined Patent Publication NO. 2001-182525 describes an exhaust gas purifying apparatus wherein plasma is generated in a plasma reactor to enhance the adsorption of NOx by a NOx adsorbent therein, when a NOx purifying catalyst located downstream thereof is at a low temperature and does not have enough NOx purifying ability. Once the catalyst in warmed up, the trapped NOx can be desorbed and purified at the downstream NOx purifying catalyst by stopping plasma.
The prior art has partially overcome the problem that the NOx purifying catalyst does not have enough NO, purifying ability before it is warmed up. However, the further improvements are required because the NOx purifying ability is not always enough even if the prior art is employed.
Therefore, the present invention provides an improved exhaust gas purifying apparatus, which overcome the above problems, and a method for using it.